The present invention relates to a control for independent parallel AC to DC converters supplying a DC load or the use of two current controlled inverters connected to an AC motor.
Paralleling two current controlled inverters, each inverter being supplied by its own phase controlled rectifier, to supply an AC machine has the advantages of reducing current harmonics, torque pulsations and eddy current losses. Each of the parallel inverters needs only half the power rating of the machine being supplied, reducing the size of the commutating capacitors and the ratings of the controllable switching devices of the inverter. A three phase inverter has a six pulse torque ripple with the currents having an approximately 120.degree. duration and rectangular shape. Two parallel inverters provide a twelve pulse ripple since there is a 30.degree. phase shift introduced by advancing the firing of one inverter relative to the other. The currents provided by the parallel inverter is more sinusoidal in shape than the six step current wave.
A microcomputer control of a twelve pulse inverter simplifies the implementation of the firing controls of the inverter by eliminating analog squaring, square root, divider and multiplier circuits. It is desirable to use a single microcomputer to control the independent parallel AC to DC converters supplying the parallel inverters to reduce costs and reduce the communication complexity that multiple micro processors can introduce.
During normal steady state operation the source converter of one parallel channel is fired 30 electrical degrees after the other parallel channel. However, when one channel is to be shut down or its current brought to zero while the other channel maintains operation, a problem can arise if a single microcomputer is used to alternately perform the firing angle calculation and firing for the independent parallel AC to DC converters supplying both channels. Since the AC to DC converter of both channels will sometimes need to be fired at time intervals requiring simultaneous execution of the firing angle regulation and firing this can lead to possible transient loss of control.
It is a object of the present invention to provide a single microcomputer control for the firing angle determination of two independent parallel source converters in a 12 pulse AC motor drive.
It is a further object of the present invention to provide a control using a single microcomputer for the firing angle determination of both parallel source converters in a 12 pulse AC motor drive without creating severe restriction on the execution time of the firing angle regulators.
It is a still further object of the present invention to allow completely arbitrary firing angles for two parallel AC to DC converters supplying a load without any interference in the current regulator programs for each of the converters run by a single microcomputer.